Three-Awn Grasses Are Negative Campaigners

23Sep

Aristida sp.

Poaceae

Today John shot a new panoramic photo (gigapan) in Jonathan Dickinson State Park to continue documenting recovery after a prescribed burn. That was before we left the IPad in the woods in the thundershower. (No worries—it had a happy ending, and compelled a second trip to a lovely park.)

Monoculture. See notes on photos below.

Prescribed burning keeps much Florida acreage in a fire-dependent “early” successional stage. A recurring thought overlooking such fire savannas is the prevalence of grasses, as opposed to understory plants from other families. Several reasons, not of much interest today, help explain this, including fire-resistant rhizomes and the ability of grasses to rise from basal growth points after the tops succumb to flames, to deer, or to John Deere. Many grasses, including three-awns, have what’s called C4 photosynthis, a special advantage in hot climates.

Let’s zoom like a gigapan on one component of grassish domination… monocultures of just one grass genus, Aristida. Aristidas are a big bunch, over 300 species around the warm world. Why so successful?

To start with a fun if not important answer, their name gives a clue about one advantage, they are three-awn grasses. An awn is a bristle, and each spikelet (seed-head) has three of them. What’s so great about that?

So many great things! Herbivores probably don’t much cotton to needle salad. Moreover, associated with the “seeds” (achenes) the awns catch on fur, wind, and water to disperse. Also they may help lodge against other vegetation or take hold on the soil. That awns serve diverse functions is hinted by their diversity in abundance, lengths, orientations, postures, shapes, barbs, stiffness, and probably responses to moisture.

Awns are three, easy ID

In some species all the awns are coequals, while in others the central awn dwarfs the other two. Many additional grasses have awns, if not in triplets, and looking beyond Aristida, awns contribute to photosynthesis and help position the seed for optimal water uptake. To drift into left field, I wonder if they absorb water or nutrients.

Speaking of nutrients, how does a monospecific understory of grass secure the nitrogen it needs? In contrast with grasses, legumes and many other plants have nitrogen-fixing nodules. And frequently free-living soil microbes contribute, as does decaying organic matter.

Doing the twist

Now we could do the obvious and fret over how meadows of grass get the nitrogen they need to survive as the fittest. But turn the beat around… and take a page from the presedential politics playbook. There are two ways to get ahead, rise up on your merits (boring), or tear down the opponents.

So, in honor of 2016, how do Aristidas manage to become President of the Park? They are negative campaigners. The explanation requires some shameless speculation and extrapolation of limited research. Just read on:

Aristidas are early-succession plants, pioneers. Any ecology book tells us that pioneer species soon give way to different replacement species, these soon booted out by bigger better species, and so forth. Another name for pioneer species is weeds, and some very talented weeds are legumes with their nitrogen-fixing nodules. Legumes are all over Jonathan Dickinson Park, but not among the Aristidas. Why don’t legumes and their power-nodules move in there?

Aristidas filibuster. They expand and overstay their fair turn in the succession parade. Some of their advantages are obvious, such as those awns, and tough rhizomes, but there’s a more interesting trick up their sleeves. Three-awns repulse their would-be replacements by destroying the nitrogen-fixing bacteria critical to the interlopers. Aristidas loathe legumes, turning the pretty pink nodules to black.

How’d you like a mouthful of that?

Of course if you deprive the foe of nitrogen you do need to get your own. Three-awns can probably acquire it rapidly after a fire, but so much more intriguing is the increasing research on nitrogen-fixation by grasses with the assistance of bacteria different from the ones the grasses suppress. Purple three-awn has associated with its root sheath nitrogen-fixing bacilli belonging to, or similar to, Bacillus (Paenibacillus) polymixa, perhaps cryptically contributing to the grasses persistence in the face of competitors.

Time to wind down, but let this then be a lesson to all political hopefuls. You too can grow up to be President. Be stubborn. Be bristly. Be toxic. And most importantly, have grass-roots support.

Note on today’s photos. These are from a project John and I worked on ca. 2007. I do not recall who shot which. We know what the species are but that info is suppressed as a distraction. The essay concerns ecology, not taxonomy.

6 responses to “Three-Awn Grasses Are Negative Campaigners”

Well, yes, I think they do. Take a drive through any well managed natural area…great monocultures or low-diversity near-monocultures of three-awns in places, bluestem grass lawns in places, and saw palmetto carpets under the pines. Those self-perpetuating grasses love fires, and bounce right back up, as John is showing so clearly in the gigapan series. I’m not saying that negatively or critically…because, like root canals and final exams, you need the burns despite some discomfort. And also, you’re going to get vast expanses of single species in places without prescribed burns. So my strong hunch is that burns wind up with uniform understories, but I don’t have much of a better idea. Seems like it would be nice to let nature follow its course and not burn…but sadly enough the result would be takeover by invasive exotics, and buildup of fuel until truly nasty fires happen. So we’re kinda stuck. At least by staggering fires and careful planning you can strive for different successional stages.

There is quite a mix. Some non-legume nitrogen fixers do form nodules on non-legume dicots. And many fixers live freely in the soil. This includes cyanobacteria as well. I’ve got a funny feeling that the world of nitrogen fixers is remarkably complex and interesting, and that contemporary biotech is going to open that window showing how simplistic we’ve all been over the years. Pretty sure a chapter in that future book will be on grasses.